1. Field of the Invention
The present invention relates to a microphone with narrow directivity capable of efficiently reducing wind noise.
2. Related Background of the Invention
Conventionally, a general configuration for causing a microphone to have narrow directivity is a configuration using an acoustic tube. For example, such a configuration is used widely, in which the front end of an acoustic tube composed of a metal tube is used as an acoustic terminal and an opening provided in the circumferential wall of the acoustic tube is used as an acoustic resistor. Further, such a configuration is also used, in which an acoustic resistor is adhered to the opening. FIG. 5 shows an example of a conventional microphone with narrow directivity.
In FIG. 5, to one end (the right end in the figure) portion of a cylindrical acoustic tube 10, a microphone unit 14 is attached, and the other end portion of the acoustic tube 10 is an acoustic terminal 22. On the circumferential wall on further front side than the microphone unit 14 of the acoustic tube 10, a slit 18 to be an acoustic resistor is provided in parallel to the center axis line of the acoustic tube 10. As for sound waves from the directions other than the direction of the center axis line of the acoustic tube 10, the sound wave that enters the acoustic tube 10 through the acoustic terminal 22, which is the front end side of the acoustic tube 10, and the sound wave that enters the acoustic tube 10 through the slit 18 on the tube side interfere with each other to decrease the sound pressure level, and only the sound wave in the direction of the center axis line is converted into an electric signal in the microphone unit 14. This is the principle of a microphone with narrow directivity.
FIG. 6 shows a measurement result of the frequency characteristic of the above-mentioned conventional microphone with narrow directivity, wherein the horizontal axis represents the frequency (Hz) of sound wave and the vertical axis represents the output signal level (dBV). Based on the standards of Electronic Industries Association of Japan (hereinafter, referred to as “EIAJ”), with an output voltage at a specified frequency with a fixed sound pressure and a specified incidence angle under specified conditions being as a reference, the ratio of the reference output voltage to the output voltage by a sine wave signal is expressed in decibel as a function of the frequency. Hereinafter, the characteristic curves shown in FIGS. 2, 11, and 13 are also measured under the same conditions. A curve “a” shows the case where a location of a sound source is at 0 degree with respect to the center axis line of the acoustic tube, that is, just in front of the acoustic tube, a curve “b” shows the case where a location of the sound source is at 180 degrees with respect to the center axis line of the acoustic tube, that is, just behind the acoustic tube, and a curve “c” shows the case where a location of the sound source is at 90 degrees with respect to the center axis line of the acoustic tube, that is, just beside the acoustic tube. It can be said that the more distant the curves “b” and “c” become from the curve “a”, the higher the directivity is. FIG. 7 shows the directivity of the above-mentioned conventional microphone with narrow directivity, wherein a scale of a concentric circle corresponds to 1 dB, and the vertical direction in the figure coincides with the longitudinal direction of the acoustic tube. The standards of EIAJ apply also to the measurement of the characteristic exhibiting the directivity as shown in FIG. 7, and the free sound field sensitivity of a microphone for a specified frequency or a narrow frequency band is expressed as a function of an incidence angle of a sound wave. FIGS. 3, 12, and 14 also show the measurement results performed under the same conditions. The frequency of the sound source is 1,000 Hz. As is seen from the measurement result in FIG. 7, the directivity is relatively excellent, that is, 133 degrees.
FIG. 8 shows the measurement result of wind noise of the above-mentioned conventional microphone with narrow directivity. The wind noise is a sound other than the sound to be captured originally, caused to occur when an air flow hits and passes over the acoustic tube, and belonging to a relatively low frequency region. The magnitude of wind noise is expressed by the equivalent sound pressure level by a wind in a state in which no sound field is present with respect to a wind the velocity and direction of which are specified according to the standards of EIAJ. Specifically, the generated voltage at a wind velocity of 2 m/s is measured and the equivalent sound pressure level at this time is obtained. Here, the characteristic shown in FIG. 4 is also measured under the same conditions. In FIG. 8, the horizontal axis represents the sound wave frequency (Hz) and the vertical axis represents the microphone output level (dB). As is seen from FIG. 8, there is a drawback that the wind noise level is high and an unpleasant low frequency noise is likely to mix in.
The applicants of the present invention applied for patent about a condenser microphone with narrow directivity in which acoustic terminals are provided in the front and in the rear of a microphone unit (for example, refer to a patent document 1). FIG. 9 is a diagram schematically showing the invention described in the patent document 1. In FIG. 9, the inside of the acoustic tube 10 is partitioned into a front acoustic chamber 11 and a rear acoustic chamber 13 by the microphone unit 14, and the front acoustic chamber 11 and the rear acoustic chamber 13 are acoustically connected by a gap 15 between the outer circumferential surface of the microphone unit 14 and the inner circumferential surface of the acoustic chamber. The front end of the above-mentioned front acoustic chamber 11 is opened and comes to be the acoustic terminal 22, and a circular hole opened in the side wall of the acoustic tube 10 constituting the rear acoustic chamber 13 comes to be an acoustic terminal 24. Since the microphone is configured such that the above-mentioned gap 15 functions as acoustic impedance and the acoustic terminals 22 and 24 in the front and in the rear of the microphone unit 14 are short-circuited by the above-mentioned acoustic impedance, a sound wave of extremely low frequency such as wind noise can be reduced.
The longer an acoustic tube is, the higher the directivity becomes. On the other hand, a vibration noise of a microphone with narrow directivity depends on the mass of air in the acoustic tube, and the longer the acoustic tube, the more the mass of the air in the acoustic tube increases, thereby increasing the vibration noise as well. However, according to the invention described in the patent document 1, since the acoustic terminals in the front and in the rear of the microphone unit are short-circuited by the above-mentioned impedance, it is also possible to reduce the vibration noise.
However, there is a drawback that when an air flow hits the acoustic terminal 22 at the front end of the acoustic tube 10, low frequency wind noise is produced and unpleasant “gurgling” noise is output because the level of the frequency region of the wind noise is large. It is conceivable that the low frequency response of a microphone is electrically reduced by using a low cut circuit as a means for reducing such wind noise. However, if a low cut circuit is used, the diaphragm of the microphone unit vibrates at a low frequency due to wind, therefore, it is difficult to avoid sound modulated with a low frequency from being output. Therefore, in an extreme case, sound may be intermittent accompanied by a “gurgling” wind noise.
As described above, it is difficult to reduce wind noise while obtaining narrow directivity and various proposals have been presented in addition to the invention described in the patent document 1. For example, an attempt is made in which an acoustic resistor is attached to the acoustic terminal on the front side of an acoustic tube or the acoustic terminal on the front side is closed. FIG. 10 shows still another example of a conventional microphone with narrow directivity. In FIG. 10, the inside of the acoustic tube 10 is partitioned into the front acoustic chamber 11 and the rear acoustic chamber 13 by a unit holder 12 holding the microphone unit 14. The front end of the above-mentioned front acoustic chamber 11 is opened and comes to be the acoustic terminal 22 and the circular hole opened in the side wall of the acoustic tube 10 constituting the rear acoustic chamber 13 also comes to be the acoustic terminal 24. In the tube wall of the acoustic tube 10, at least one straight slit 18 is formed in parallel to the center axis line of the acoustic tube 10 on the front acoustic chamber 11 side. The slit 18 is covered with an acoustic resistor 20 adhered to the outer circumferential surface of the acoustic tube 10. The acoustic resistor 20 is made of cloth, non-woven fabric cloth, film, etc. Although not shown in FIG. 10, there may be the case where the acoustic terminal 22 on the front side is also covered with an acoustic resistor or the acoustic terminal 22 is closed.
FIG. 11 shows the measurement result of the output signal level (dBV) for the sound frequency (Hz) in the configuration as shown in FIG. 10, in which an acoustic resistor is attached to the acoustic terminal 22 at the front end of the acoustic tube 10. Similarly to the case shown in FIG. 6, a curve “a” shows the case where the location of a sound source is at 0 degree with respect to the center axis line of the acoustic tube, that is, just in front of the acoustic tube, a curve “b” shows the case where the location of a sound source is at 180 degrees with respect to the center axis line of the acoustic tube, that is, just behind the acoustic tube, and a curve “c” shows the case where the location of a sound source is at 90 degrees with respect to the center axis line of the acoustic tube, that is, just beside the acoustic tube. As is seen from the measurement result, in comparison with FIG. 6 that shows the measurement result of the conventional example shown in FIG. 5, the sensitivity drops on the contrary and the high frequency sound region is degraded. FIG. 12 shows the measurement result of the directivity of the one having such a configuration as shown in FIG. 10 in accordance with FIG. 7. As is seen from FIG. 12, the directivity is also degraded in comparison with the conventional example shown in FIG. 5.
FIG. 13 shows the measurement result of the output signal level (dBV) for the sound frequency (Hz) in the configuration as shown in FIG. 10, in which the acoustic terminal 22 at the front end of the acoustic tube 10 is closed in accordance with FIG. 6. As is seen from this measurement result, the sensitivity and the directional frequency response in the sound band are degraded. Further, FIG. 14 shows the measurement result of directivity in the above-mentioned configuration in accordance with FIG. 12. As is seen from FIG. 14, the sound level that enters from the rear side increases and the directivity is also degraded.
[Patent document 1] Japanese Patent Application Laid-Open No. 2000-83292